1. Field of the Disclosure
The present disclosure relates generally to wireless communication and, more particularly, to wireless communication in unlicensed frequency bands.
2. Description of the Related Art
Unlicensed frequency bands are portions of the radiofrequency spectrum that do not require a license for use and may therefore be used by any device to transmit or receive radio frequency signals. For example, the Unlicensed National Information Infrastructure (UNII) is formed of portions of the radio spectrum that include frequency bands in the range of 5.15 GHz to 5.825 GHz such as the U-NII-1 band in the range 5.15-5.25 GHz, the U-NII 2a, b, c bands in the range 5.25-5.725 GHz, and the U-NII 3 band in the range 5.725-5.825 GHz. Unlicensed frequency bands can be contrasted to licensed frequency bands that are licensed to a particular service provider and may only be used for wireless communication that is authorized by the service provider.
Wireless communication devices that transmit or receive signals in licensed or unlicensed frequency bands are typically referred to as nodes, which may communicate according to different radio access technologies that are defined by different standards. For example, nodes may include Wi-Fi access points that operate according to IEEE 802.11 standards in the unlicensed spectrum. For another example, nodes may include base stations that operate in licensed spectrum according to standards such as Long Term Evolution (LTE) standards defined by the Third Generation Partnership Project (3GPP).
Base stations that operate according to LTE may implement supplemental downlink (SDL) channels in the unlicensed spectrum to provide additional bandwidth for downlink communications to user equipment that are also communicating with the base station using channels in a licensed frequency band. For example, one or more primary LTE carriers are anchored in the licensed spectrum and may be aggregated with one or more secondary LTE carriers in the unlicensed spectrum when the base station and one or more user equipment operate in a carrier aggregation mode. Wi-Fi access points also use unlicensed spectrum for wireless communication.
In order to take advantage of the bandwidth provided in the licensed spectrum and the unlicensed spectrum, user equipment can implement multiple radios for concurrent access to interfaces in the licensed frequency bands and the unlicensed frequency bands according to different radio access technologies. For example, user equipment may implement a radio to support an LTE interface in the licensed spectrum (LTE-L), a radio to support an LTE interface in the unlicensed spectrum (LTE-U), and a radio to support a Wi-Fi interface in the unlicensed spectrum. The different radios must be sufficiently isolated from each other to prevent collisions of transmitted or received signals. For example, a typical Wi-Fi transmit power of a smart phone is approximately 18 dBm and the in-band blocking requirement for LTE is −30 dBm, which implies that the Wi-Fi transmitter and the LTE receiver must be isolated from each other by at least 48 dBm. Frequency separation can provide sufficient isolation between the licensed frequency bands and the unlicensed frequency bands. However, there is no guarantee that signals transmitted by different nodes in the unlicensed frequency bands will be isolated from each other by frequency separation.